Thanks Jim, I will use your data when I correct the other error. I have to say that I have been uncomfortable with the 10 times approximation between snow and ice as being too trite. Can I plead ignorance as we in Australia don't see a lot of snow unless we travel to our few mountains. Our highest mountain, Mount Kosciusko, is only 2228 metres high somewhat puny by your standards.
Cheers, Pat Naughtin Geelong, Australia On 2011/02/10, at 07:32 , James R. Frysinger wrote: >> If anybody is worried about their roofs, please use at least 320 kg/m³ for >> wet snow on sloped roofs. If you have a flat roof, determine whether the >> drain is working, it will be the difference between 320 kg/m³ wet snow and >> 960 kg/m³ slush. > > I believe that those figures assume an accumulation of 80 cm of "snow", > whether it be light and fluffy, wet and dense, slushy, or icy. If 80 cm of > snow falls, then, due to insolation (not insulation!) and warming by air or > by conduction from below, the depth will no longer be 80 cm. > > And of course, not all snow accumulations are 80 cm in depth. > > Rather than recommending a load estimate figure (that is based on 80 cm of > accumulation of whatever sort -- snow, slush, ice), it might be preferable to > teach the method, which then can be adapted to any given precipitation amount. > > I collect and report daily precipitation data for CoCoRaHS > http://www.cocorahs.org > including snowfalls. For snow I measure the depth of the accumulation, > collect snow from an area of known size, and weigh it to determine the > "rainfall equivalent". Working backwards for this winter for some of our > snowfalls, I observed that the snow:rain ratio might be better stated as > 1 cm:0.7 mm > 1 cm:0.75 mm > 1 cm:1.2 mm (followed rain and freezing rain) > 1 cm:0.26 mm (notes indicate unusually light & fluffy snow > with large "flakes") > > As you can see, there is quite a bit of variation in those ratios for my > location. Generally, I would tend to characterize our snows as averaging 0.7 > mm to 0.8 mm rainfall equivalent in 1 cm of snow from what I have seen the > last 4 years. As John suggests, other areas might typically see snow of a > different average density. > > This variation is exactly why meteorologists melt (or weigh) fallen and > accumulated snow to determine its actual water content. If one is concerned > and capable enough to estimate roof loading, they probably should do likewise. > > By the way, some architects might show maximum snow loadings on the plans for > the structures built from those plans. > > Jim > > On 2011-02-09 1208, John M. Steele wrote: >> I just received Metrication Matters 93, and saw the aircraft hanger >> example again. As it is snow season in the US, and people need to worry >> about their roofs, I have to point out two huge errors in the example as >> I don't believe anyone should rely on that example. >> 80 cm of snow != 8 mm of rain >> Even if the 10% rule were true, it would imply 80 cm of snow is 80 mm of >> rain. Doing some Googling on snow load and roof designs, I find the >> density of wet, heavy snow is more like 32-33% water density, 320 - 330 >> kg/m³. The Washington (DC) area is not noted for light, fluffy powder, >> and light fluffy powder isn't what collapses roofs. Using the 320 kg/m³ >> x 0.8 m, the actual roof load was more like 256 kg/m³ if the drainage >> system was still working, not the 8 kg/m² of the worked example. >> Flat roofs are a particular problem as snow tends to clog drainage and >> then you get slush, a mixture of ice and water. Not surprisingly, the >> density of slush lies between 920 kg/m³ (ice) and 1000 kg/m³ (water). >> The figure above of 320 kg/m³ is for drained (but wet) snow - imagine >> snow on a screen so any water melt can drip out. >> I can't find the spec for flat roofs, a lot of local codes in the >> northern US are 35-40 lb/ft² for sloped roofs. That converts to 170 >> kg/m². Some extreme snow areas are higher, and I would expect flat roofs >> to be higher. >> If anybody is worried about their roofs, please use at least 320 kg/m³ >> for wet snow on sloped roofs. If you have a flat roof, determine whether >> the drain is working, it will be the difference between 320 kg/m³ wet >> snow and 960 kg/m³ slush. >> > > -- > James R. Frysinger > 632 Stony Point Mountain Road > Doyle, TN 38559-3030 > > (C) 931.212.0267 > (H) 931.657.3107 > (F) 931.657.3108 > Pat Naughtin LCAMS Author of the ebook, Metrication Leaders Guide, see http://metricationmatters.com/MetricationLeadersGuideInfo.html Hear Pat speak at: http://www.youtube.com/watch?v=_lshRAPvPZY PO Box 305 Belmont 3216, Geelong, Australia Phone: 61 3 5241 2008 Metric system consultant, writer, and speaker, Pat Naughtin, has helped thousands of people and hundreds of companies upgrade to the modern metric system smoothly, quickly, and so economically that they now save thousands each year when buying, processing, or selling for their businesses. Pat provides services and resources for many different trades, crafts, and professions for commercial, industrial and government metrication leaders in Asia, Europe, and in the USA. Pat's clients include the Australian Government, Google, NASA, NIST, and the metric associations of Canada, the UK, and the USA. See http://www.metricationmatters.com for more metrication information, contact Pat at [email protected] or to get the free 'Metrication matters' newsletter go to: http://www.metricationmatters.com/newsletter to subscribe.
